1. Field of the Invention
The present invention relates to an endless core for a multiphase transformer and a transformer incorporating such a core.
2. Description of the Related Art
Multiphase transformers are well known and are used in a variety of applications including for stepping up or stepping down line voltage in power transmission systems, to provide phase shifting, modulation, star-delta converters and general power supplies.
A typical multiphase transformer has a planar core provided with a number of square or rectangular windows each window being bound by upper and lower branches of the core, and on opposite sides by vertical legs forming part of the core. A primary winding is wound through each window, either on a branch or leg of the window. Similarly a secondary winding is wound through each window. Irrespective of the number of phases, if the core has N windows then it will have N+1 vertical legs. This provides an inherent magnetic and therefore electrical imbalance between the phases. This arises because the magnetic flux created by current flow in the primary windings cannot circulate equally about the respective windows because of the additional vertical leg. As a result, assuming each primary phase voltage is of the same magnitude and each secondary winding has the same number of turns, then the secondary outputs cannot be the same. The transformation process is not identical between the phases due to the difference in magnetic paths surrounding each window. In order to produce equalized outputs on the secondary windings, i.e. the same magnitude output on each winding, some of the primary or secondary windings must vary the number of turns to take account of the difference in flux distribution circulating about different windows of the transformer core. Such transformers also have inherent inefficiencies due to flux leakage caused by the exposed, dead end nature of the core and the end windows having only a single oscillating flux path.